This invention is directed to the operation of an ethylene dichloride ("EDC") cracking furnace such as is used to produce vinyl chloride ("VC") monomer from EDC feed. During operation the equipment, if not made from a suitable alloy, is subject to corrosion, and particularly the furnace, though fitted with alloy tubes, is subject to coking. When this occurs, the furnace, and the preheater for feed to the furnace, must be shut down and cooled. The preheater is then manually, laboriously cleaned, and the furnace decoked.
Since the commercial production of vinyl chloride monomer ("VCM") is a continuous operation in which a typical furnace cannot economically produce less than about 100 million pounds per year (MM lb/yr) of VCM, it will be appreciated that shutting down the furnace for any reason, no matter how compelling, is not happily tolerated.
It has recently been found that only a trace, as little as 10 parts per million (ppm) by weight of iron present as ferric chloride (FeCl.sub.3) will effectively force a 100 MM lbs/yr cracking furnace to be shut down after only about three weeks of operation. The FeCl.sub.3 is typically generated in ferrous metal pipes through which EDC is fed to the furnace, or to a lesser extent, may be carried over from a catalytic reactor in which EDC is generated by the reaction of ethylene and chlorine in the presence of FeCl.sub.3 catalyst.
The problem of maintaining less than the trace quantity of FeCl.sub.3 in the EDC is exacerbated because (a) the liquid phase chlorination of ethylene requires a FeCl.sub.3 catalyst, and (b) the reactor in which the EDC is produced is made of a ferrous metal, particularly carbon (mild) steel, for economic reasons. When a high nickel alloy such as Hastelloy, Inconel, or a titanium or glass-lined reactor is used, the only FeCl.sub.3 to be contended with is the entrained catalyst, which is not difficult. If the effluent from the alloy or titanium reactor is led through non-ferrous or glass-lined piping to non-ferrous or glass-lined storage tanks, the problem of coking due to FeCl.sub.3 in the tubes of the EDC furnace essentially disappears. But even in an all-alloy plant, if 20 ppm or more free chlorine is present in the feed to the EDC furnace, the tubes coke up.
When the stored EDC is preheated before it is fed to the EDC furnace, and such heating is done in a ferrous metal heat exchanger (preheater), the corrosion and coking problems are revived. If the feed to the furnace is preheated in an alloy preheater, and there is 20 ppm or more of free chlorine present, the coking problem is revived.
Stated differently, the problem of corrosion in piping, storage tanks, heat exchangers, etc., and coking of the tubes of an EDC cracking furnace can be effectively negated only by carefully guarding against the presence of either 30 ppm of FeCl.sub.3, or 20 ppm free chlorine, or both, in the feed to the furnace. Because the cost of building an all alloy EDC plant is economically difficult to justify, as much of the equipment as possible is constructed with ferrous metals, particularly carbon steel. More specifically, since the cost of a boiling reactor used in the high temperature direct chlorination (HTDC) of ethylene militates in favor of a carbon steel one, the problems of corrosion in the equipment train, and coking of the EDC furnace are both problems to which an economical solution is sought.
Therefore this invention is most particularly directed to minimizing the corrosion in an EDC plant, and particularly the coking of the EDC cracking furnace in a plant where the EDC is generated in a boiling reactor reactor made from ferrous metal such as carbon steel; where, even if an alloy reactor is used, the EDC produced is stored in carbon steel storage tanks; or where, even if made in an alloy reactor and stored in an alloy storage tank, the liquid EDC feed to the furnace contains 20 ppm or more of free chlorine which EDC is vaporized in a carbon steel preheater; in any of which situations, the sensitivity of the equipment to corrosion due to free chlorine, and coking of the furnace due to the presence of trace quantities of either FeCl.sub.3 or free chlorine, becomes of prime importance.
In view of the specificity of the problem stated hereinabove, this invention is of most value in an EDC plant where the boiling reactor provides the driving force for the feed to the EDC furnace. Such a reactor is operated at the boiling point of EDC, typically under pressure of up to about 50 lb/in.sup.2 gauge (psig), under a wide range of other operating variables (i) to minimize the entrainment of FeCl.sub.3 catalyst in the reactor; (ii) to minimize the production of unwanted byproducts; and, (iii) to maintain as low an excess of ethylene over stoichiometric as is practical to minimize the amount of unreacted (free) chlorine in the effluent from the reactor. Further, since excess ethylene cannot be economically recovered, any such ethylene is not only wasted but `rides` through the system at considerable cost. The deceptively simply stated goal is to convert all the chlorine fed to the EDC reactor with a minimum excess of ethylene, and to avoid forming as little as 30 ppm of FeCl.sub.3 (10 ppm as Fe, and about 20 ppm as Cl) in the feed to the EDC reactor.
As is well known, the economics of chemical engineering unit operations in the production of EDC from VC monomer are such that, optimally, the ethylene and chlorine are converted to EDC without the formation of unwanted byproducts and most important, without leaving any free chlorine residue in the effluent. The problem of corrosion is discussed in "Alloy Selection for VCM Plants" by Schillmoller, C. M., Hydrocarbon Processing pg 89-93, March 1979.
In practice, economics dictate that the direct chlorination reaction be controlled so that carbon steel equipment may be used. The problem is that as little as from about 20 ppm to about 60 ppm of free chlorine in carbon steel equipment and piping upstream of the EDC reactor has a highly corrosive effect on its tubes. The problem is further magnified when trace amounts of moisture in the range from 10 ppm to about 50 ppm are also present.
In the course of culling the numerous variables to select those which critically affect the viability of the commercial process, it was further discovered that the "make" of unwanted byproducts was a function of the temperature at which the boiling reactor operates, the higher the temperature the greater the make. This relationship dictated that the boiling reactor be operated at as low a pressure as was practicable.
To minimize the amount of unreacted chlorine leaving the reactor (referred to herein as "free" or "breakthrough" chlorine), an excess of ethylene is supplied to it. By "excess ethylene" we refer herein to ethylene in an amount greater than that stoichiometrically required to produce EDC. However, even when more than 2% excess ethylene is supplied, the amount of free chlorine in the effluent remains in the range from about 100 ppm to about 3000 ppm, and substantially all of it has to be removed. Thus, after having selected the critical variables it was necessary to tailor each one within narrow limits which would effectively provide the results sought, namely desirably coke-free operation of the EDC cracking furnace.
We do not know of any prior art reference which has recognized, much less addressed the problem a trace quantity (from 30 ppm to about 100 ppm) of FeCl.sub.3 presents in an EDC furnace. We are well aware that the problem of minimizing corrosion due to the effluent, without specific regard as to minimizing the production of free chlorine and its effect on process equipment in an EDC plant, has confronted many persons skilled in the art. Corrosion is pronounced even at room temperature; it gets exponentially worse, doubling for every 10.degree. C. increase, so that in the range above 50.degree. C. it is in full effect; and, if one wishes to operate a commercial boiling reactor, one cannot avoid operating in the elevated temperature range.
To minimize corrosion in the equipment generally, in such a manner as to provide an effluent which is not only acceptably corrosive but economically not unduly burdensome is a difficult problem to which a better solution is constantly sought; but, to do so with specific regard to the trouble-free low coking operation of an EDC furnace adds to the difficulty of solving the problem. Part of the difficulty lies with the varied considerations which define the problem, as it presents itself in different guises, hence the elusiveness of the solution; and by no means a minor part lies in the unforgiving economics of any solution to the problem. It is axiomatic that solutions to industrial problems must be economically acceptable.
It is well known that FeCl.sub.3 is an addition catalyst which catalyzes the chlorination of ethylene, of EDC, and of VC; and, the formation of ethyl chloride by the addition of HCl to VC. During operation of an oxychlorination reactor Shiozaki et al in U.S. Pat. No. 4,329,323 teach that FeCl.sub.3 from the reactor "may transpire from the reactor or cause troubles such as choking of the reactor" (col 1, lines 51-52). They recognized that the catalyst itself might lead to an unacceptably high pressure drop. Their problem was to remove ethylene and VC simultaneously; and, to do so they inject chlorine which must be present in excess (up to 15 mol % excess). When they inject the chlorine they created the problem which we were to address. Shiozaki et al were unconcerned with the effectiveness of the EDC furnace, nor did they recognize that trace quantities of free chlorine would vitiate its effectiveness. Neither were they concerned with the formation of ethyl chloride and/or 1,1,2-trichloroethane (`triane`). To cope with the pressure drop they used catalyst having arbitrary geometry but an outer surface area per unit packed catalyst volume of not less than 7.8 cm.sup.2 /ml. This catalyst creates too high a pressure drop if used to remove chlorine from the effluent of a boiling reactor.
U.S. Pat. No. 4,029,714 to Ziegenhagen et al teaches a process analogous to that described by Shiozaki et al, in which a chlorine-removal system comprising a heat exchanger, a fixed bed reactor (referred to herein as a "polishing reactor") and a separator, is placed immediately down-stream of an ethylene clean-up system, and the combination is operated at a greater chlorine-to-ethylene feed ratio than the up-to-10% molar excess with respect to ethylene (typically used). Like Shiozaki et al, they stressed the effectiveness of a supported ferric chloride catalyst in combination with metallic iron, but concluded the effectiveness of the process was predicated upon a choice of the proper ratio of the superficial area of the iron to the total BET surface area of the alumina, without regard to catalyst geometry or contact time.
Like the Shiozaki et al process, the '714 process recognized the problem of a very long catalyst bed contributing to a high pressure drop and specifically selected a catalyst with sufficient activity to avoid the problem, not recognizing that extended contact times would favor formation of ethyl chloride, etc., or that generation of FeCl.sub.3 in the bed by reaction of chlorine with the iron, itself could create enough of a pressure drop to choke the reactor. But pressure drop was not critical except if it approached a level threatening to choke the reactor. Extended contact time and relatively high pressure drop are unrelated to the operation of the oxychlorination reactor in which the EDC is generated because, unlike a boiling reactor, the operation of an oxychlorination reactor is far less sensitive to increased pressure.
Thus in each of the foregoing '323 and '714 processes, ethylene is fed to an ethylene clean-up reactor along with at least enough chlorine to react with it, the ethylene feed containing HCl and chlorinated hydrocarbons from an oxychlorination reactor. The '714 reference teaches that the cleaned-up chlorine-rich stream, with as much or as little chlorine as is left unreacted, is then led to the polishing reactor, where, given a long enough bed of an activated alumina catalyst impregnated with FeCl.sub.3, low levels of ethylene and chlorine may be reached in the polished effluent. But the contact time would be so great as to convert valuable EDC and VC to triane and ethyl chloride respectively, inter alia, neither of which can be economically recovered, and the pressure drop would be so high as to preclude the operation of a boiling reactor operating at about 600 MM lb/yr rate of EDC production.